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Date:   Mon, 15 Oct 2018 09:07:36 -0700
From:   Nathan Chancellor <natechancellor@gmail.com>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Andrew Lutomirski <luto@kernel.org>,
        nick.desaulniers@gmail.com,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86/mm: annotate no_context with UNWIND_HINTS
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On Mon, Oct 15, 2018 at 09:03:40AM -0700, Andy Lutomirski wrote:
> On Mon, Oct 15, 2018 at 8:31 AM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >
> > On Mon, Oct 15, 2018 at 08:22:21AM -0700, Nathan Chancellor wrote:
> > > > >>> @@ -760,9 +760,11 @@ no_context(struct pt_regs *regs, unsigned long error_code,
> > > > >>>                 * and then double-fault, though, because we're likely to
> > > > >>>                 * break the console driver and lose most of the stack dump.
> > > > >>>                 */
> > > > >>> -               asm volatile ("movq %[stack], %%rsp\n\t"
> > > > >>> +               asm volatile (UNWIND_HINT_SAVE
> > > > >>> +                             "movq %[stack], %%rsp\n\t"
> > > > >>>                              "call handle_stack_overflow\n\t"
> > > > >>> -                             "1: jmp 1b"
> > > > >>> +                             "1: jmp 1b\n\t"
> > > > >>> +                             UNWIND_HINT_RESTORE
> > > > >>>                              : ASM_CALL_CONSTRAINT
> > > > >>>                              : "D" ("kernel stack overflow (page fault)"),
> > > > >>>                                "S" (regs), "d" (address),
> > > > >>
> > > > >> NAK.  Just below this snippet is unreachable();
> > > > >>
> > > > >> Can you reply with objtool -dr output on a problematic fault.o?  Josh,
> > > > >> it *looks* like annotate_unreachable() should be doing the right
> > > > >> thing, but something is clearly busted.
> > > > >>
> > > > >> Also, shouldn't compiler-clang.h contain a reasonable definition of
> > > > >> unreachable()?
> > > > >>
> > > > >> --Andy
> > > > >
> > > > > Hi Andy,
> > > > >
> > > > > Did you mean 'objdump -dr'? If so, here you go (rather long, sorry if I
> > > > > should have pasted it here instead):
> > > > > https://gist.github.com/nathanchance/f038bb0a6653b975bb8a4e64fcd5503e
> > > > >
> > > > >
> > > >
> > > > Hmm, -dr wasn’t quite enough to dump the .discard bits, assuming they’re there at all. Can you just put the whole .o file somewhere?
> > >
> > > Here you go: https://nathanchance.me/downloads/.tmp/fault.o
> >
> > $ eu-readelf -S /tmp/fault.o  |grep reachable
> > [12] .discard.reachable   PROGBITS     0000000000000000 00002bc0 00000014  0        0   0  1
> > [13] .rela.discard.reachable RELA         0000000000000000 00002bd8 00000078 24 I     32  12  8
> >
> > That confirms that you need a clang version of the unreachable() macro.
> >
> 
> Duh.
> 
> That being said, the generic macro is:
> 
> # define unreachable() do { annotate_reachable(); do { } while (1); } while (0)
> 
> I'm probably missing some subtlety here, but shouldn't that be
> annotate_*un*reachable()?
> 
> Of course, there are any number of reasons why there should be a real
> definition.  Nathan and Nick, does adding something like:
> 
> #define unreachable() \
>         do {                                    \
>                 annotate_unreachable();         \
>                 __builtin_unreachable();        \
>         } while (0)
> 
> to compiler-clang.h fix the problem?
> 
> --Andy

Ha, I was just typing out a message summarizing that that exact
definition fixed this warning.

Nathan